My 10 yo son likes rocks (because they change color in the water). I just say they have to be big enough not to accidentally go down the drain. He has also taken cork tops from wine/champagne bottles into the bath. He has tried cardboard tubes and paper boats (very carefully colored with wax crayons to stop the water) into the bath (total mess). We also have these plastic beakers and I buy the soap samplers of different colors and types. He is always "testing" the bubbles/color mixes et cetera. He also likes the play shark, submarine with diver man and octopus and plays out discovery channel stuff. Usually ending in a lot of yelling "help" and dead diver man. If you find something great, let me know. BTW, they do make waterproof radios for the shower.